Fertilizers for Iron

Formation of barely soluble iron hydroxides and oxides, particularly at high pH and in the presence of bicarbonate ions in the rooting medium, immobilizes iron supplied as inorganic salts. One way round this problem is to supply Fe(III) citrate, but this is photolabile. For these reasons the supply of iron in hydroponic culture is usually as a chelate (27). This can be as either FeEDTA (ethylenediaminetetraacetate) or FeEDDHA (ethylene diamine (di o-hydroxyphenyl) acetate). Both these chelates remain stable over a range of pH values, particularly FeEDDHA, although the iron is readily available to the plants. In fact, the whole chelate molecule can be taken up at high application rates, and as this absorption is by a passive mechanism it is probably at the root zone where the lateral roots develop (106). However, the main uptake of iron chelates in soils or nutrient solutions at realistic application rates takes place after exchange chelation in Strategy 2 plants (48) and after Fe(III) reduction and formation of Fe₂⁺ in Strategy 1 plants (107). Interestingly, cucumber plants supplied with inorganic Fe seem to be more resistant to infection by mildew than plants supplied with FeEDDHA (106).

In terms of fertilizers for terrestrial plants, iron deficiency usually comes about because of alkaline pH in the soil, and supply of iron salts to the soil would have no effect. Foliar application of Fe(II) sulfate can be effective, typically as a 1% solution applied at regular intervals (25).

Where iron deficiency occurs in acid soils, supply of Fe(II) sulfate to the soil can be effective. Thus in ornamental horticulture, azaleas and other acid-loving plants benefit from application of this salt. However, in the field, supply to citrus trees on acid soils is not effective as other ions, particularly copper, interfere with the availability of iron (25). Application of iron can be made as FeEDTA or FeEDDHA, but the stability of FeEDTA at least is not high in calcareous soils (25). FeEDDHA and FeDTPA are the only commercially available iron chelates for soil application because of their stability at high pH. The synthetic iron phosphate vivianite (Fe3(PO₄)₂� 8H₂O) has been used on olive trees (108) and in kiwi orchards (109).

Therefore, the usual way in which lime-induced chlorosis is alleviated is by supply of iron chelates such as FeEDTA and FeHEDTA to the foliage. Usually more than one application is required (110). There is potential for supplying iron to the foliage of plants as iron-siderophores, as these microbial chelates are more biodegradable than the synthetic chelates, and so pose less environmental risk (111). FeEDTA may also damage the leaves of plants. It is also possible that these microbial siderophores could be used for root application, at least in hydroponics, as iron-rhizoferritin and Fe(III) monodihydroxamate and Fe(III) dihydroxamate siderophores have been shown to be taken up by a range of plant species by exchange chelation with phytosiderophores or via Fe(III) reduction in Strategy 2 and Strategy 1 plants, respectively (48,112,113).

Some of the effects of lime-induced chlorosis on the early stages of plant growth can be overcome by planting seeds that are high in iron. In the case of common bean (Phaseolus vulgaris L.), seeds from plants grown on acid soils are higher in iron than seeds from plants grown on calcareous soils, but the seed iron content can be increased by supply of iron to the soil at planting or after flowering (114). A preplanting application of FeEDDHA has a larger effect on seed yield of soybean (Glycine max L.) than an application at flowering, but the latter application has a more beneficial effect on iron concentration in the seeds of both common bean and soybean (115). There is other evidence that the iron concentration in soybean seeds is under very tight genetic control and is not influenced much by the supply of iron, but in that experiment the FeEDDHA was supplied at planting (116).